First results from the Chara Array. III. Oblateness, rotational velocity, and gravity darkening of alderamin

We present observations of the A7 IV-V star Alderamin (alpha Cep, HR 8162, HD 203280) from the Georgia State University CHARA Array. These infrared interferometric angular size measurements indicate a noncircular projected disk brightness distribution for this known rapid rotator. The interferometric observations are modeled as arising from an elongated rigid atmosphere, with apparent polar and equatorial radii of r(p) 0: 6753(-)(0.0135)(+)(0.0119) and r(e) (+)(0.0293)(-)(0.0183) mas, respectively, for a difference of 201 +/- 32 mu as, and with an axial ratio of r(e)/r(p) 1.298 +/- 0.051. Using the Hipparcos distance of 14: 96 +/- 0.11 pc, these angular measures translate to 2.18 +/- 0.05 and 2.82 +/- 0.10 R-circle dot. The inclination of Alderamin to the line of sight indicated by this modeling is effectively edge- on (i = 88.2(-)(13.3)(+) (1.8) deg). The star has a true rotational velocity of 283 +/- 10 km s(-1) (similar to 83% of breakup velocity) and a polar temperature of roughly 8400 K. Significantly, a necessary aspect of this modeling is a determination of the gravity-darkening coefficient, which at a value of beta = 0: 084(-)(+) (0.026)(0: 049) is consistent with a convective photosphere, as expected for an A7 IV-V star. Our detailed characterization of this object allows us to investigate various scenarios for the angular momentum history of Alderamin and the appropriateness of certain stellar evolution models.